An exciting finding in the first funding cycle of this grant is that skeletally immature animals have a functionally successful repair response to ACL injury, whereas adolescent and adult animals do not. We postulate that the impaired ACL healing noted in adolescents and adults could be improved if the fundamental biologic processes of increased cellular migration and proliferation found with immature ACL cells and repair tissue could be recapitulated in the adolescent animals using our novel enhanced ACL repair strategy. In this competitive renewal, we will determine if the impaired healing potential of adolescent ACL tears can be improved in vivo by using an enhanced repair strategy in which a scaffold stimulates increased cellular density of the repair tissue by facilitating fibroblast migration and proliferation within the wound site. We propose to study the effects of these additives in both the ideal acute repair situation, and in the more clinically relevant case in which the repair is delayed by 2, 4 or 6 weeks in the following two Specific Aims: Aim 1: Evaluate the effect of increasing the cellular density of the ACL repair tissue on functional healing after an immediate repair of a ruptured adolescent ACL. Hypothesis 1a: Tissue engineered scaffolds that increase the cellular density of the ACL repair tissue will have a higher yield load and linear stiffness and lower AP knee laxity after 15 weeks of healing than repairs using scaffolds which impede cellular population of the wound site. Hypothesis 1b: An increase in cell density is related to an increase in yield load and stiffness and a decrease in AP knee laxity. Hypothesis 1C: The repairs augmented with a tissue engineered scaffold containing platelets or erythrocytes will have a higher yield load and linear stiffness and lower AP knee laxity than repairs with collagen alone or ACL reconstruction. Aim 2: Establish the effect delaying repair on the functional healing of the ACL. Hypothesis 2a: The delay of primary repair by 2, 4 or 6 weeks will result in changes in the yield load and linear stiffness of an enhanced ACL repair relative to the acute condition. Hypothesis 2b: A greater degree of cartilage and meniscal damage will be present in knees undergoing repair at 6 weeks as compared to the immediate repair situation. For both Aims our well-characterized adolescent mini pig model of ACL injury and repair will be utilized. The primary outcome measures will include biomechanics (AP knee laxity, ligament structural properties) and histology (cellular density and vascular density). Secondary outcome measures include cartilage injury (India ink staining and histology of the menisci and articular cartilage) and the material properties of the repair. The studies in the proposal are designed to determine whether the impaired ACL healing noted in adolescents and adults could be improved if the repair tissue in these populations could be made to more closely resemble the repair tissue in immature animals using our novel enhanced ACL repair strategy. Success in this endeavor could improve the care of hundreds of thousands of adolescent and adult patients with ACL tears each year.